GRB 990704: the most X-ray rich BeppoSAX gamma-ray burst

We present the Xand γ-ray detection of GRB 990704 and the discovery and study of its X-ray afterglow, 1SAX J1219.5-0350. Two pointed BeppoSAX observations with the narrow field instruments were performed on this source, separated in time by one week. The decay of the X-ray flux within the first observation appears unusually slow, being best-fit by a power law with negative index 0.83±0.16. Such a slow decay is consistent with the non-detection in our second observation, but its back-extrapolation to the time of the GRB largely underestimates the detected GRB X-ray prompt emission. In addition, the GRB prompt event shows, among the BeppoSAXWFC detected sample, unprecedentedly high ratios of Xand gamma-ray peak fluxes (F2−10 keV /F40−700 keV∼0.6, and F2−26 keV /F40−700 keV∼1.6) and fluences (S2−10 keV /S40−700 keV∼1.5 and S2−26 keV /S40−700 keV∼2.8), making it, among the BeppoSAX arcminute-localized GRBs, the closest to the recently discovered class of Fast X-ray Transients. 2 M. Feroci et al.: GRB 990704: the most X-ray rich BeppoSAX GRB